1. Field of the Invention
The present invention relates to a variable speed motor, and more particularly to an outer rotor-type variable speed motor to which single phase AC power is applied, wherein the frequency of AC power applied to the motor is detected and a relay is provided to allow main windings provided in the motor to be connected in series or parallel to each other depending on whether the detected frequency is high or low, so that the motor operates to suit the frequency characteristics of the single-phase AC power.
2. Description of the Related Art
FIG. 1 is an exploded perspective view showing a rotor and a stator of a general outer rotor motor.
A general motor includes a stator on which windings are mounted, and a rotor on which a permanent magnet, an aluminum conductor, or a core is formed. The motor is a device that causes periodic changes in current flowing through the windings mounted on the stator and produces torque in the rotor due to periodic changes in magnetic field caused by the changes in the current, thereby attaining rotation power.
Motors are generally divided into inner and outer rotor motors according to the positions of stators and rotors. The outer rotor motor has a structure in which a rotor is positioned outside a stator as shown in FIG. 1 so that the outer rotor is rotated by changes in current flowing through windings of the stator.
Since rotors of outer rotor motors are provided outside stators, outer rotor motors have a larger turning radius than inner outer motors. Accordingly, outer rotor motors increase torque per unit volume compared to inner rotor motors. Thus, without requiring expensive start devices, outer rotor motors can be used for load for which high start torque is needed. Outer rotor motors can also be embodied to be more compact in structure than general inner rotor motors in the case where they need to produce the same torque.
General single phase induction motors require start devices, which are generally divided into split-phase start, shading coil, capacitor run, repulsion start types, etc.
FIG. 2 is a circuit diagram showing a general capacitor-run single-phase induction motor.
The general capacitor-run single-phase induction motor includes a main winding M1, a subsidiary winding S1, and a capacitor C1 connected in series to the subsidiary winding S1. When single phase AC power E1 is applied to the motor, the main winding M1 produces an alternating magnetic field and the subsidiary winding S1 produces a subsidiary magnetic field that is 90 electrical degrees out of phase with the alternating magnetic field produced by the main winding M1.
Since the alternating magnetic field produced by the main winding M1 and the subsidiary magnetic field produced by the subsidiary winding are 90 degrees out of phase, the two magnetic fields are not canceled but combined to generate a rotating magnetic field, thereby rotating the single-phase induction motor.
However, the general single-phase induction motor described above has a problem in that the motor speed varies depending on the frequency of AC power applied to the motor.
The rotation speed of the induction motor is determined by an equation “n=120f/p”, where “f” is the frequency and “p” is the number of poles. The number of poles of a motor is set to a specific value when the motor is manufactured. Therefore, if a motor, which is designed to operate at a rated power frequency of 50 Hz, is used with 60 Hz power, the rotation speed of the motor is increased by 20% compared to when the motor is used with 50 Hz power.
To prevent an increase in the speed of the motor when the motor operates with 60 Hz commercial power, conventionally, an inverter circuit for changing the frequency of the applied power is attached to the motor, or an applied voltage on/off control method is employed, or a decelerator or a belt or gear is used to mechanically reduce the rotation speed of the motor.
However, these conventional methods increase the manufacturing cost of the motor or cause noise due to the electrical on/off control of the applied power or due to the mechanical deceleration, thereby reducing the efficiency of the power.
On the other hand, in the case where the conventional capacitor-run single-phase induction motor operates at a rated frequency, there may be a need to change the rotation speed of the motor to a low or high speed to suit the operating mode when the motor is applied to household electrical appliances.
When the single phase induction motor operates at the rated frequency, the motor is maintained at a constant speed corresponding to the point at which motor and load torque curves, which will be described later, meets each other, and therefore a separate apparatus must be provided to allow the speed control.
However, if the separate apparatus is provided, the manufacturing cost of the motor is increased. To overcome this problem, a pole-change single-phase induction motor, which has a relatively low manufacturing cost, is conventionally used as a motor speed control device.
FIG. 3 is a view showing the structure of a conventional 2 pole-4 pole change single-phase induction motor.
As shown in FIG. 3, the pole change single-phase induction motor includes four windings, i.e., a 2-pole main winding (1a, 1b), a 2-pole subsidiary winding (2a, 2b), a 4-pole main winding (3a, 3b, 3c, 3d), and a 4-pole subsidiary winding (4a, 4b, 4c, 4d) which are wound on the stator through slots thereof. The rotation speed of the motor is varied by driving the motor using the 2-pole main winding and the 2-pole subsidiary winding in 2-pole running mode and driving the motor using the 4-pole main winding and the 4-pole subsidiary winding in 4-pole running mode.
However, the conventional pole-change single-phase induction motor configured as described above has problems in that the use of the four windings for changing the number of poles increases the sectional area of the slots, significantly reducing the efficiency of the motor due to an increased core loss of the stator, and it is also difficult to increase the variable speed range since the achievable minimum variable speed is limited.